1. Field of the Invention
This invention relates to processes for the manufacture of ethanol by fermentation.
2. Description of the Prior Art
With the ever-increasing depletion of economically recoverable petroleum reserves, the production of ethanol from vegetative sources as a partial or complete replacement for conventional fossil-based liquid fuels becomes more attractive. In some areas, the economic and technical feasibility of using a 90% unleaded gasoline-10% anhydrous ethanol blend ("gasohol") has shown encouraging results. According to one study, gasohol powered automobiles have averaged a 5% reduction in fuel compared to unleaded gasoline powered vehicles and have emitted one-third less carbon monoxide than the latter. In addition to offering promise as a practical and efficient fuel, biomass-derived ethanol in large quantities and at a competitive price has the potential in some areas for replacing certain petroleum-based chemical feedstocks. Thus, for example, ethanol can be catalytically dehydrated to ethylene, one of the most important of all chemical raw materials both in terms of quantities consumed and versatility.
The various operations in processes for obtaining ethanol from such recurring sources as cellulose, cane sugar and amylaceous grains and tubers, e.g., the separation of starch granules from non-carbohydrate plant matter and other extraneous substances, the chemical and/or enzymatic hydrolysis of starch to fermentable sugar (liquefaction and saccharification), the fermentation of such sugar to a dilute solution of ethanol ("beer") and the recovery of anhydrous ethanol by distillation, have been modified in numerous ways in an attempt to achieve improvements in product yield, production rates, and so forth. The substitution of alcohol for at least a portion of petroleum based fuels is particularly critical for developing economies where proven domestic petroleum reserves are limited, such as in India and Brazil, and these nations have therefore increasingly emphasized the production of alcohol from vegetative sources. However, for biomass-derived ethanol to realize its vast potential as a partial or total substitute for petroleum fuels or as a substitute chemical feedstock, it is necessary that the manufacturing process be as efficient in the consumption of raw material and energy as possible if there is to be a significant energy return for the amount of ethanol produced and if the ethanol is to become an economically viable alternative to petroleum based raw materials. To date, however, at most only modest attention has been directed to optimizing raw material and energy consumption in the manufacture of ethanol from biomass.
Processes for the continuous fermentation of sugars to provide alcohol are well known (viz., U.S. Pat. Nos. 2,155,134; 2,371,208; 2,967,107; 3,015,612; 3,078,166; 3,093,548; 3,177,005; 3,201,328; 3,207,605; 3,207,606; 3,219,319; 3,234,026; 3,413,124; 3,528,889; 3,575,813; 3,591,454; 3,705,841; 3,737,323; and 3,940,492; "Process Design and Economic Studies of Alternative Fermentation Methods for the Production of Ethanol", Cysewski, et al. Biotechnology and Bioengineering, Vol. xx, pp. 1421-1444 (1978)). Several known fermentation processes are known in which a combination of fermenting organisms are used (viz., U.S. Pat. Nos. 2,182,550; 2,202,785; 2,431,004; 2,419,960; 2,529,131; 2,567,257; and 3,093,548). In a typical continuous fermentation process, a stream of sterile sugar liquor and a quantity of yeast cells are introduced into the first of a battery of fermentation vessels wherein initial fermentation takes place, generally under conditions favoring rapid cell growth. The partial fermentate admixed with yeast cells is continuously withdrawn from the first fermentation vessel and introduced into a second fermentation vessel wherein fermentation is carried out under conditions favoring the rapid conversion of sugar to ethanol. The yeast in the last fermentation vessel can be recovered by suitable means, e.g., centrifugation or settlement, and recycled. It has been discovered that in such a system, the typically high concentrations of sugar which are present in the first fermentation vessel inhibit the growth and productivity of the yeast. A further drawback of conventional fermentation processes lies in their inability to effectively convert all or most of the sugar oligomers and repolymerizates to ethanol. Such oligomers and repolymerizates tend to resist conversion by yeasts which are commonly employed in known fermentation procedures. This disadvantage is particularly a problem when the sugar employed in the fermentation is derived from the acid hydrolysis of carbohydrate polymer, e.g., starch. Three of the principal types of undesirable reactions known to take place in acid catalyzed carbohydrate polymer hydrolysis are: degradation wherein the starch molecule is irreversibly destroyed to provide 5-hydroxymethylfurfural which hydrolizes to levulinic acid and formic acid, and separately to humins; reversion wherein the product glucose repolymerizes and/or isomerizes to unfermentable substances; and retrogradation wherein hydrolysis splits out the branched chain components of the starch molecule leaving a straight chain, lower molecular weight water-insoluble polymeric molecule which crystallizes at about 70.degree.-80.degree. C. and becomes resistant to further hydrolysis. To the extent these and other undesirable reactions take place, they cause the production of sugar derivatives which undergo fermentation to ethanol only with difficulty, if at all.
Accordingly, there has heretofore existed a need for a process of rapid, efficient continuous fermentation of fermentable sugar and sugar oligomers and/or repolymerizates such as those derived from the acid hydrolysis of starch to provide industrial ethanol at competitive prices.